Python 2.7 could be significantly slower (5x in some cases) when compiled with clang 3.x or 4.x, compared to clang 5.x. This is quite a problem on macOS, since the latest clang from Apple (which comes with Xcode 9.2) is based on LLVM 4.x. This issue was first noticed by Bart Skowron and reported to the Homebrew project.[1]

I ran some preliminary benchmarks (here[2] are the exact setup scripts) with just a simple loop:

    import time

    def f(n):
        while n > 0:
            n -= 1

    start = time.time()
    f(50000000)
    stop = time.time()
    print('%.6f' % (stop - start))

and here are my results:

- macOS 10.13.2 on a MacBook Pro:

    2.082144	/usr/bin/python2.7
    7.964049	/usr/local/bin/python2.7
    8.750652	dist/python27-apple-clang-900/bin/python2.7
    8.476405	dist/python27-clang-3.9/bin/python2.7
    8.625660	dist/python27-clang-4.0/bin/python2.7
    1.760096	dist/python27-clang-5.0/bin/python2.7
    3.254814	/usr/local/bin/python3.6
    2.864716	dist/python-master-apple-clang-900/bin/python3
    3.071757	dist/python-master-clang-3.9/bin/python3
    2.925192	dist/python-master-clang-4.0/bin/python3
    2.908782	dist/python-master-clang-5.0/bin/python3

- Ubuntu 17.10 in VirtualBox:

    1.475095	/usr/bin/python2.7
    8.576817	dist/python27-clang-3.9/bin/python2.7
    8.165588	dist/python27-clang-4.0/bin/python2.7
    1.779193	dist/python27-clang-5.0/bin/python2.7
    1.728321	dist/python27-gcc-5/bin/python2.7
    1.570040	dist/python27-gcc-6/bin/python2.7
    1.604617	dist/python27-gcc-7/bin/python2.7
    2.323037	/usr/bin/python3.6
    2.964338	dist/python-master-clang-3.9/bin/python3
    3.054277	dist/python-master-clang-4.0/bin/python3
    2.734908	dist/python-master-clang-5.0/bin/python3
    2.490278	dist/python-master-gcc-5/bin/python3
    2.494691	dist/python-master-gcc-6/bin/python3
    2.642277	dist/python-master-gcc-7/bin/python3

I haven't got time to run more rigorous benchmark suites (e.g., the performance[3] package). I did try the floating point benchmark from performance, and again saw a 2x difference in performance.

[1] https://github.com/Homebrew/homebrew-core/issues/22743
[2] https://gist.github.com/zmwangx/f8151ba8907ba8159a07fdd1528fc2b5
[3] https://pypi.python.org/pypi/performance